The invention relates to a cathode-ray tube comprising in an evacuated envelope an electron gun for generating an electron beam, which electron gun has a cathode unit comprising a cup-shaped cathode shaft and a cathode filament disposed therein, comprising a thin metal wire wound in the form of a first helical spiral, which is folded or wound into a second spiral and is covered with insulation material, the ends of the first spiral being welded to connection braces.
The invention also relates to a cathode unit for such a cathode-ray tube.
Such cathode-ray tubes have a very wide field of application and are used, for example, as television camera tubes, television display tubes, oscilloscope tubes, and the like.
Such a cathode-ray tube is described in Netherlands patent application No. 8103814 corresponding to U.S. patent application Ser. No. 401,456 filed July 26, 1982. The filament of the cathode unit described in this patent application is connected by means of laser welding to two connection braces manufactured from flat sheet metal. Laser welding is to be preferred over resistance welding because in resistance welding the filament is touched during the welding process and welding spatters may occur causing short circuits in the tube. However, when laser welding is used during mass production of cathode units it has been found that the resistances of their respective filaments differ considerably. This results in differing heating properties and hence differing emissions of the cathodes.